In a typical imaging and analysis system a target (or sample) is scanned in two dimensions by a probe signal. A corresponding detected signal provides information about the scanned points in the target which can then be displayed as an image and analyzed visually or can be analyzed electronically by an electronic processing system.
Sub-surface imaging is a powerful technique for non-destructive imaging or quantitative analysis of a variety of targets (also referred to as samples) including, semiconductor wafers, materials, human tissue, etc. The analysis can include scanning for defects, discontinuities, or in the case of tissue, abnormalities such as malignant growths.
High resolution sub-surface imaging is particularly valuable in the case of in vivo analysis of human tissue, where, non-destructive, non-invasive sub-surface scanning allows convenient analysis of potentially abnormal tissue without the need for a costly, time consuming and invasive biopsy.
Two dimensional scanning of imaging systems typically consists of scanning in one dimension along one direction or axis then, at a lower speed, scanning or stepping in a direction orthogonal to the first direction. Scanning is typically accomplished by electro mechanical devices, such as galvanometers or moving coils actuators. Other scanning technologies include rotating polygons which are expensive, physically large and have significant alignment issues or acousto-optic (AO) scanners which are expensive, require significant RF power and, since the angular deviation is small, involve systems that are physically large.
A typical sub-surface imaging technology, such as confocal microscopy, can generate tomographic images, for example of tissue, containing information similar to biopsy sections, by scanning a one dimensional array, parallel to the surface of the tissue (x-scan), at varying depths (z-scan) in tissues. The series of one dimensional scans at various depths can be displayed as a single tomographic image. High resolution is achieved by having a high numerical aperture (NA) focusing lens. Such lenses have an undesirable trade off between working distance and physical size of the lens. To achieve both high resolution and long working distance require large and therefore very expensive lenses. These systems also typically have undesirable moving parts or expensive and high power consumption AO modulators.
Another sub-surface imaging technology, optical coherence tomography, can also generate tomographic, biopsy like images. Such systems use a Super-luminescence diode (SLD) as the optical source. The SLD output beam has a broad bandwidth and short coherence length. Optical coherence tomography involves splitting the output beam into a probe and reference beam. The probe beam is applied to the system to be imaged or analyzed (the target). Light scattered or reflected back from the target is combined with the reference beam to form the measurement signal.
Because of the short coherence length only light that is scattered or reflected from a depth within the target such that the total optical path lengths of the probe and reference are equal combine interferometrically. Thus the interferometric signal provides a measurement of the scattering or reflection value at a particular depth within the target. By varying the length of the reference path length, a measurement of the scattering values at various depths can be measured and in this manner, the z-axis can be scanned. The reference path length is typically varied by physically moving a reflecting mirror.
In this case, high resolution in one dimension is achieved by using an SLD with a broad wavelength range (or large spectral width). This is typically limited by the properties of the material comprising the SLD, which represents a limitation on the achievable resolution. Because z dimension scanning requires varying the path length, at least some of the above mentioned limitations, such as mechanical moving parts or expensive AO modulators, apply to this imaging method also and, in general, these limitations represent a barrier to applying current imaging technologies to compact, cost effective high resolution applications.
Furthermore, SLDs emit incoherent light that consists of amplified spontaneous emissions with associated wide angle beam divergence which have the undesirable beam handling and noise problems. The beam is also a continuous wave (CW) source with no opportunity for temporal based signal enhancement. Also, because of the random nature of spontaneous emission, the reference signal must be derived from same SLD signal and have equal optical path length as the probe signal. Therefore the relative optical path length must be physically changed by a scanning mechanism and the reference path length must be of similar magnitude to the probe path length. These aspects cause systems based on SLD sources have significantly lower signal to noise characteristics and present problems in practical high resolution implementations.
In general one or more of these aspects of high cost components, moving parts, high power consumption and large physical size make existing imaging systems unsuitable for cost effective, compact, robust, high resolution imaging systems. There is therefore an unmet need for a cost effective, compact, robust, high resolution sub-surface imaging or analysis system.